Field of the Invention
The present invention relates in general to detecting a hydraulic brake circuit failure within a split vehicle braking system, and more particularly, to detecting a hydraulic brake circuit failure while requiring a pressure sensor in only one of the two hydraulic brake circuits.
Description of the Related Art
Vehicle braking systems commonly include a master cylinder and reservoir for providing pressurized brake fluid to hydraulic brake circuits for actuating vehicle brakes. Vehicle braking systems typically include two hydraulic brake circuits for actuating a respective set of vehicle brakes. During vehicle stability control mode, as a driver of the vehicle asserts a force on the brake pedal of a vehicle, the master cylinder (M/C) pressure is typically measured by a respective pressure sensor in each hydraulic brake circuit or other type of sensing device which determines the M/C pressure within each of the hydraulic brake circuits (e.g., pressure switch or a brake pedal travel sensor measuring the distance the brake pedal has traveled) for determining the drivers braking demands. Based on the braking demands received from the sensing devices, a motor, pump, and associated valves provide pressurized hydraulic brake fluid to the vehicle brake actuators for actuating the vehicle brakes. The pressurized hydraulic brake force applied to the vehicle brake actuators is directly correlated to the driver's braking demands (the M/C pressure as measured in both hydraulic circuits).
Some vehicle braking systems may include secondary assist brake functions. Such secondary assist brake functions provide anti-lock braking (ABS), traction control (TC), and yaw stability control (YSC) functions. These secondary assist brake functions supplement the driver actuated hydraulic brake system. For example, the ABS system pulsates the braking system if the operator of the vehicle locks the brakes so as prevent the vehicle from skidding and to shorten the braking distance traveled of the vehicle. Each of the assisted brake functions provides some type of added vehicle braking or vehicle stability control utilizing either one or more brakes individually or in combination. The secondary brake assist functions require additional pressure above the driver applied M/C pressure. For example, for the YSC function, the M/C pressure is used as a reference for the pressure target of an isolated circuit. During YSC control, pressure for an isolated circuit is required to be increased above the M/C pressure (i.e., YSC+M/C pressuremeasured). A pressure controller and pressure estimate is used to deliver the required pressure to the wheels of the isolated circuit for applying the secondary assist brake function. These functions typically are automatically activated usually without awareness of the driver.
When a hydraulic brake circuit failure is detected, secondary assist braking functions, such as anti-lock braking, traction control, yaw stability control, may be deactivated for allowing the operator of the vehicle to slow down or stop the vehicle on its own without any assistance from secondary assist braking functions. The reason is to prevent any automated braking actions, which are activated by the secondary assist braking functions, from interfering with the driver's intended braking demands. For example, if the hydraulic brake failure is occurring, and the driver wants to stop immediately, the assisted braking cannot sense this issue or condition the driver is faced with and may attempt a vehicle stability control operation that is not in cooperation with the drivers intended braking operation.
To detect a hydraulic brake circuit failure a pressure sensor is provided for each hydraulic brake circuit in the vehicle. Typically there are two hydraulic brake circuits for providing hydraulic brake fluid to a respective wheel or sets of wheels. In a two hydraulic brake circuit system, each hydraulic brake circuit provides hydraulic brake fluid to a respective pair of vehicle brakes and two pressure sensors are often utilized. Alternatively, in the two hydraulic brake circuit system, a pressure sensor could be utilized on a first hydraulic brake circuit and a pressure switch could be utilized on the second hydraulic brake circuit. Each pressure sensor or switch provides a signal indicative of the fluid pressure within a respective circuit.
To properly utilize the assisted brake functions, it must be known whether both hydraulic brake circuits are functioning properly, otherwise the assisted brake functions could ignore the drivers intended braking demands. If only one pressure sensor was utilized within a braking system utilizing two hydraulic brake circuits, then such a failure could go undetected. For example, if a failure occurred in the sensed circuit, the pressure sensor measuring pressure on the sensed line would provide a M/C pressure measurement of approximately zero. The unsensed circuit, in this example, would have a M/C pressure greater than zero. However, without a second sensor (or other type of indicator) in the unsensed circuit for comparing the pressure within the two hydraulic brake circuits, a potential fault condition may go undetected and braking system would not recognize the drivers braking demands as the M/C pressure is zero in the sensed circuit. As a result, minimum or no braking force would be applied to the vehicle brakes in YSC control modes.
In the event of a failure of an unsensed line, the pressure sensor would measure the pressure applied within the sensed circuit (non-failed circuit) and would provide a signal to the motor, pump and associated valves to supply pressurized hydraulic brake fluid to the vehicle brake actuators. However, zero or minimal pressure will be generated in the unsensed (failed) circuit, and as a result, only a portion of the vehicle braking would be applied.
The hydraulic brake failure would remain undetected in both situations without some secondary method of verifying the hydraulic brake failure, secondary assist brake functions would remain active. Such braking function could potentially conflict with the driver's immediate braking demands. Thus, in YSC systems, it is critical to utilize a pressure sensor in each hydraulic brake circuit, or other device (e.g., pressure differential switch) to detect the failure, however, the addition of a pressure sensor for each additional circuit in a braking system becomes costly.